Dr.Bob, yesterday you told me :
"The best way I think is to show examples.
12Mg = 1s2 2s2 2p6 3s2
Here the 1s orbital is lower than the 2s and they are lower than the 3s etc."

19K = 1s2 2s2 2p6 3s2 3p6 3d0 4s2
Here the 1s is lower than the 2s and those are lower than the 3s and the 4s BUT the 4s level is lower than the 3d. " for what I wrote below but isn't the electron config for 19K is 1s2 2s2 2p6 3s2 3p6 4s1 ?

The orbitals of a principal energy level are lower in energy than the orbitals in the next higher principal energy level.

Yes, you are right. I got carried away with typing the numbers. But the example is still valid and what I said about it still is valid. I put the 3d0 in there to emphasize that the 3d orbitals are not filled UNTIL the 4s orbital is filled.
19K is [Ar]4s1--after 3p6 we have 3d then 4s. The 4s fills before anything goes into 3d. For example, see Ca and Sc.
What you wrote, in what I assume is an explanation, is not true all the time. The original question as I remember was, "Is this true all the time or some of the time" and my example was to show that it was true some of the time but not all the time.
20Ca is [Ar]4S2
Then 21Sc is [Ar} 3d1 4s2